JJ THOMSON

Joseph John Thomson

Who was JJ Thomson ?

His discovery of the electron

Bio

Details

His work

His discovery of isotopes

Career

Details

His collaborators

The plum pudding model

Description

Names

Thomson's work

In 1897, Thomson showed that cathode rays were composed of previously unknown negatively charged particles (now called electrons), which he calculated must have bodies much smaller than atoms and a very large charge-to-mass ratio. Thomson is also credited with finding the first evidence for isotopes of a stable (non-radioactive) element in 1913, as part of his exploration into the composition of canal rays (positive ions). His experiments to determine the nature of positively charged particles, with Francis William Aston, were the first use of mass spectrometry and led to the development of the mass spectrograph.

His discovery of the electron

In 1897, J.J. Thomson discovered the electron, a subatomic particle much smaller than an atom, through experiments with cathode rays. Prior to this, scientists like William Prout and Norman Lockyer had speculated about fundamental units of atoms but believed them to be hydrogen-sized. Thomson found that cathode rays (negatively charged particles) were more than 1,000 times lighter than hydrogen atoms and consistent across different types of atoms, indicating a universal component of matter. He initially called these particles "corpuscles," though "electron," coined by George Johnstone Stoney in 1891, became the preferred term.Thomson demonstrated that cathode rays could be deflected by electric fields when the discharge tube was at a low pressure, allowing him to measure the mass-to-charge ratio of the electron. This established a method of analysis that was confirmed in subsequent experiments. In 1904, Thomson proposed the "plum pudding model" of the atom, where electrons were embedded in a sphere of positive charge. Despite initially resisting the term "electron," Thomson adopted it by 1914. The positively charged counterpart was named the "proton" in 1920, following Ernest Rutherford's work.

Who was JJ Thomson?

• J.J. Thomson was born in Cheetham Hill, the suburb of Manchester, on December 18, 1865. • He attended Owens College in 1870 and Trinity College in 1876 • He became Master of Trinity College in 1918 and remained there until his death. • He died on August 30, 1940 and was buried in Westminster Abbey, close to Sir Isaac Newton.

The discovery of isotopes

In 1912, while investigating the composition of positively charged canal rays, J.J. Thomson and his assistant F.W. Aston directed a stream of neon ions through magnetic and electric fields, recording the deflection on a photographic plate. They observed two distinct patches of light, indicating two separate paths of deflection. This finding revealed that neon consisted of atoms with two different atomic masses, neon-20 and neon-22, providing the first evidence for isotopes of a stable element. Frederick Soddy had earlier suggested isotopes to explain the behavior of radioactive elements. Thomson's work marked the first use of mass spectrometry, a technique later refined by F.W. Aston and A.J. Dempster.

Ernest Rutherford

Henry Moseley

Niels Bohr

Known as the father of nuclear physics, Rutherford was a student of Thomson and worked with him on various studies related to the atom. Later, he made groundbreaking contributions to the understanding of atomic structure and radioactivity, including the discovery of the atomic nucleus.

• Moseley worked as a researcher under Thomson's guidance and made key contributions to the development of atomic physics. Moseley is famous for Moseley's Law in X-ray spectra, which provided a systematic method for determining the atomic number of elements.

Bohr worked with Thomson for a brief period in the early stages of his career, and Thomson's research influenced Bohr's development of the Bohr model of the atom, which was a significant advancement in quantum theory.

The plum pudding model was the first scientific model of the atom to describe an internal structure. It was first proposed by J. J. Thomson in 1904 following his discovery of the electron in 1897, and was rendered obsolete by Ernest Rutherford's discovery of the atomic nucleus in 1911. The model tried to account for two properties of atoms then known: that there are electrons, and that atoms have no net electric charge. Logically there had to be an equal amount of positive charge to balance out the negative charge of the electrons. As Thomson had no idea as to the source of this positive charge, he tentatively proposed that it was everywhere in the atom, and that the atom was spherical. This was the mathematically simplest hypothesis to fit the available evidence, or lack thereof. In such a sphere, the negatively charged electrons would distribute themselves in a more or less even manner throughout the volume, simultaneously repelling each other while being attracted to the positive sphere's center. Thomson's model is popularly referred to as the "plum pudding model" with the notion that the electrons are distributed uniformly like raisins in a plum pudding. Neither Thomson nor his colleagues ever used this analogy. It seems to have been coined by popular science writers to make the model easier to understand for the layman. The analogy is perhaps misleading because Thomson likened the positive sphere to a liquid rather than a solid since he thought the electrons moved around in it.

The plum pudding model